Standby power supply systems are used for providing electric power for a consumer arrangement in a local power distribution grid in the case where the superordinate, possibly public power supply grid cannot provide this power. Reasons for this can be, for example, a failure of and/or a fault on the superordinate power supply grid. The standby power supply system comprises a disconnection device which, during normal operation of the superordinate power supply grid, connects the power supply grid to the local power distribution grid. In the event of a failure of and/or a fault on the superordinate power supply grid the disconnection device disconnects the local power distribution grid from the superordinate power supply grid. After disconnection of the local power distribution grid the local power supply device connects onto this local grid in order to supply power to the consumer arrangement. The local power generation device may be an inverter, which is coupled to an energy storage device, for example a battery. Such standby power supply systems for switching from a normal operating mode in the case of supply by the superordinate power supply grid to island operation in the case of supply by the local power generating unit are known, for example, from the documents EP 1 956 483 A1 and DE 20 2010 008 123 U1.
Such local power supply devices generally have an island detector in order to detect the island situation of the local power distribution grid and to implement the above-described steps. Methods for detecting such an island situation (“Anti-Islanding Detection”, AID) are known, for example from the documents EP 2 003 759 A1 and EP 0 810 713 B1.
Regarding such standby power supply systems, for safety reasons it is necessary to avoid a situation in which the local power supply device supplies power to the local power distribution grid while the grid is still connected to the superordinate power supply grid. In such a situation, the safety of personnel performing maintenance work on the actually voltage-free superordinate power supply grid would no longer be ensured, for example. Therefore usually so-called contactors with priority control and a checkback contact are provided as switching elements in the grid disconnection device. By these contactors it is ensured that the checkback contact is closed when and only when all of the main contacts are actually disconnected. The checkback contact is connected to the local power supply device so that the local power supply device only supplies power to the local power supply grid when the checkback contact is closed. In addition, often two series-connected contactors are provided in order to ensure additional safety by virtue of redundancy. Compared with conventional contactors which are not provided with priority control and which are also referred to as “installation contactors”, the mentioned contactors with priority control are firstly more expensive and secondly, owing to their size, cannot be used with conventional installation boxes (“service entrance boxes”, “domestic subdistribution cabinet”) used in domestic installation.